It's not a question of them expiring. It's that the company issuing them had an opportunity to rebuy them at cost. I.e. your vesting schedule is completely meaningless. It's one of those situations where someone uses a word that has a specific set of associations (i.e. "I vest 25%, that means I get them right?") and then attaches terms that make it mean something else entirely ("yes, you get them, but I can take them away at my option without compensating you").